1. Field of the Invention
The present invention relates to an image data reading apparatus which is capable of obtaining image data from an original document through the operation of a photoelectric conversion means. More particularly, the invention is concerned with an image data reading apparatus having a shading correction function.
2. Related Background Art
Hitherto, image data reading apparatus have been known in which the image formed by light transmitted through a transparent color original is color-separated into three primary colors such as red (R), green (G) and blue (B), and three types of image signals corresponding to respective colors, derived from an image pickup device, are converted into digital image data signals.
These known apparatus suffer from the disadvantage that the quality of the image data is degraded due to unevenness or lack of uniformity in the read image data due to various reasons such as uneven illumination of the transparent color original by the lighting system, and vignette by the projection system which projects the image of the transparent color original to the image pickup device.
In order to obviate this problem, it has been proposed to effect on the read image data a correction corresponding to any unevenness of the illumination intensity. This method, however, encounters a problem in that, when the original is read during a change in magnification, the extent of the unevenness in the illumination intensity is changed by the magnification of the projection lens so that the correction value obtained with the original cannot be used directly.
This problem would be overcome by arranging the unevenness in the illumination intensity to be measured after determination of the magnification so as to enable the correction value to be changed each time the magnification is changed.
However, a problem is still encountered particularly in the case where only a part of the image frame is cut out and read or when the projecting magnification is to be changed. Namely, in such a case, a preferred process includes projection of the image on the finder system and trimming of the projected image through a visual check. However, when the original image is projected on the finder system, the unevenness in the illumination intensity cannot be measured because the transparent original is placed in the path of the light, that the correction value cannot be determined precisely.
Another problem encountered by the conventional apparatus is that the correct measurement of unevenness in the illumination intensity is often not obtained due to dust, flaw or other contaminants attaching to the original holder or the optical system. Any dust attaching to the lighting optical system and the projection optical system, as well as dust attaching to the original film or a scratch and other defects in the film, appear as black dots or lines in the image data, thus impairing the quality of the image data read from the original.
FIGS. 2A and 2B schematically show the manner in which the image data and the output image are influenced by dust and defects, as observed in the cases where the film is a reversal (positive) film and a negative film, respectively.
As will be seen from these Figures, any dust or defect appear as black dots on the image signals when the original film is read through scanning by a film scanner followed by conversion into image signals, regardless of whether the film is a reversal (positive) film or a negative film.
In the case where the film is a reversal (positive) film, the image signals are delivered to the output device, e.g., a printer through an image processing such as .gamma. correction, so that the influence of the dust and defects directly appear as black dots in the output image, as will be seen from FIG. 2A.
On the other hand, as shown in FIG. 2B, when the film is a negative film, the image signals read by the film scanner are subtracted from the image signals which are obtained by reading at full level, thus effecting conversion from negative image to positive image so that the dust and defects cause white bright spots.
In order to eliminate these problems, British Patent Nos. 1547811 and 1547812 propose an image pickup apparatus in which the original image is scanned by infrared rays so as to detect any dust and defect which may exist on the original. In these British Patents, a dichroic mirror for separating visible rays from infrared rays is disposed in the image pickup optical path so as to separate the light beam thereby effecting correction of the image signals. The solutions proposed by these Patents, however, are still unsatisfactory in that they cannot predict the size or extent of the dust and defect, so that the they cannot effect a high degree of correction.
It is true that IC memories of a high level of performance have become available commercially, by virtue of the progress in the field of semiconductor technology. However, the price of the image pickup device will become very high if such IC memories are used for storing image data of respective colors such as R(red) G(green), B(Blue) and IR(infrared) at a high level of resolution.
In addition, the image pickup apparatus of the kind described heretofore known do not have any function which would automatically discriminate any dust and defect and give signs indicative of the presence of such dust and defect. Hitherto, therefore, it has been necessary to visually check the original film before the image pickup operation.
Actually, however, it is extremely difficult to detect tiny dust and tiny defects which may exist on the original, and it is often experienced that the operator overlooks such tiny dust and defects so that he is aware of the presence of such dust or defect only after the reproduction of the image. In such a case, it is necessary to conduct the image pickup operation once more, after removing such dust and defects, resulting in a loss of time which is serious particularly when a long time is required for the image pickup operation.
The solutions proposed by the above mentioned British Patents employ a sensor for detecting dust and defect through scanning with infrared rays, in addition to the sensor which is used for the purpose of picking up the image. When any defect is detected, the portion of the image data corresponding to the position of such a defect is replaced by image data in the region around such a defect. If the positions of both sensors are not aligned with each other, therefore, the detected position of the defect is undesirably deviated from the actual position of the defect.
Another problem is that the size of the region in which the defect exists is reduced from the actual size during binary coding of the image signals.
A solution to this problem is proposed in British Patent No. 1547812 mentioned above. In this Patent, the detected defective region is magnified by an OR circuit and a shift register circuit both in the directions of main scanning and sub scanning.
In order to electrically increase the size of the detected defective region, it is necessary to employ a circuit of a considerably large scale. In addition, in the case of an image pickup apparatus in which main scanning and subscanning are conducted independently, the defective region cannot be detected perfectly unless the subscan is conducted.
The solutions proposed by the British Patents mentioned above suffer from another disadvantage in that there is no means of recording the positions of the defective picture elements so that it is impossible to judge what portion of the original image has been corrected.
It is to be noted also that tiny dust and elongated dust can hardly be removed though large dust can be removed without difficulty at time of the original. Elongated defects also are often observed but no effective measure has been proposed hitherto for removing such defects having an elongated pattern.